1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or a copy machine, and more particularly, to an image forming apparatus of an electrophotographic system using a dry-type two-component developing method.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic system according to the related art, a surface of a photosensitive drum 1 serving as an electrostatic latent image bearing member is uniformly charged by a charging roller 2 as illustrated in FIGS. 1 and 2. The charged photosensitive drum 1 is exposed by an exposure apparatus 3 according to image information, and an electrostatic latent image is formed on the photosensitive drum 1.
For the electrostatic latent image formed on the photosensitive drum 1, a toner image is visualized by toner of a developer using a developing device 4. A transfer device 5 transfers the visualized toner image onto a recording material 10. Thereafter, a fixing device 6 fuses and fixes the toner image transferred onto the recording material 10 by applying heat and pressure to the recording material 10. A cleaning device 7 removes toner remaining on the photosensitive drum 1 after the above-described transfer process to prepare for the next image forming process.
A two-component developer having non-magnetic toner particles (hereinafter referred to as “toner”) and magnetic carrier particles (hereinafter referred to as “magnetic carriers”) is used as a developer for the developing device 4. In particular, a color image forming apparatus 100 is widely used because of, for example, good color tone since a magnetic material may not be included in the toner.
There is an issue in that a sleeve ghost phenomenon occurs when the toner within the developing device 4 is consumed by image formation and an image is formed by toner replenishment for the consumed toner. Here, the sleeve ghost is a phenomenon in which a shape of a toner image formed in a first rotation of a developing sleeve 44 during image formation appears in a second rotation of the developing sleeve 44.
In the related art, the sleeve ghost is generated when a ratio of toner particles having small particle diameters to those having an average particle diameter of the toner is high. The toner particles having the small particle diameters may be attracted by a pullback potential difference due to a potential difference between a white background potential on the photosensitive drum 1 and a surface potential of the developing sleeve 44. It is difficult to separate the toner particles from the surface of the developing sleeve 44 because an adhesion force is large due to a reflection force for the small particle diameters.
Thus, an amount of toner in the second rotation is increased by an amount of toner not supplied for development of the first rotation and directly attached to the surface of the developing sleeve 44. Thereby, a charging amount of toner with the small particle diameter is decreased by newly supplied toner and easily subjected to development in which the reflection force is small, so that chargeability for a latent image potential on the photosensitive drum 1 of the second rotation is improved and a toner density is increased to appear as an image defect.
On the other hand, Japanese Patent Laid-Open No. 2006-065317 discusses technology for defining a percentage by number of particles of which a volume-average particle diameter of toner is less than or equal to 4 μm because the sleeve ghost is easily generated if a proportion of particles of which the volume-average particle diameter of toner is less than or equal to 4 μm exceeds 8 percent by number.
However, as another factor in which the sleeve ghost is generated, a toner amount around a charging amount “0” is increased and a charging amount distribution of toner within a developing container 41 becomes broad. As a factor in which the charging amount distribution of toner becomes broad, a difference may occur between a charging amount of toner within the developing container 41 and a charging amount of replenishment toner. For example, a developer within the developing container 41 during image formation in a high-temperature, high-humidity environment has a smaller charging amount than the developer in a normal-temperature, normal-humidity environment. For example, when a toner bottle filled with the toner is used in the normal-temperature, normal-humidity environment, the charging amount of the replenishment toner in the beginning of supply is larger than that of original toner within the developing container 41.
Thus, once a magnetic carrier is in contact with replenishment toner, an electrostatic adhesion force is high and therefore the magnetic carrier is still in contact with the replenishment toner. Because the original toner within the developing container 41 in which the charging amount is small has a small electrostatic adhesion force with the magnetic carrier, it does not easily adhere to the magnetic carrier covered with the replenishment toner. Thereby, when the developer is supplied to the developing sleeve 44, toner having a small charging amount not attached to the magnetic carrier is attached to the surface of the developing sleeve 44 by a pullback potential difference. This toner serves as under toner. Here, the under toner refers to a toner layer formed on the developing sleeve 44 and attached onto the surface of the developing sleeve 44 to which a magnetic brush formed by the developer is not attached.
In addition, conversely, when an image forming apparatus is provided in a low-humidity environment, relative humidity of replenishment toner is higher than that within the developing container 41. Thus, when original toner within the developing container 41 covers the magnetic carrier, there is little opportunity for the replenishment toner to make contact with the magnetic carrier. In particular, an increase in the under toner due to toner having the low-charging amount for which an opportunity to make contact with the magnetic carrier is small easily occurs when a toner coverage rate of the magnetic carrier is high and a T/D ratio is high. Here, the T/D ratio is a mixture ratio between the non-magnetic toner and the magnetic carrier within the developing container 41, and refers to a ratio of non-magnetic toner weight (T) to total weight (D) of the magnetic carrier and the non-magnetic toner.
Thereby, the toner is developed on a latent image part drawn by laser in a toner image formed in the first rotation of the developing sleeve 44, and an amount of generation of the above-described under toner is small. In addition, toner is in a state in which the force is applied in a surface direction of the developing sleeve 44 all the way during image formation due to a pullback potential difference in a non-image formation part, not the latent image part. Thus, the under toner is easily generated, and the low charged toner or the uncharged toner serves as the under toner and is adsorbed onto the surface of the developing sleeve 44 because the reflection force with the magnetic carrier is weak. Thereby, an amount of toner to be supplied in a developing process in the second rotation of the developing sleeve 44 is biased at a latent image position by the under toner generated in the first rotation, and a sleeve ghost image is generated.
The present invention provides an image forming apparatus capable of reducing a sleeve ghost caused by a charging amount difference between a developer and replenishment toner within a developing device when temperature/humidity of a replenishing container is different from that within the developing container.